Erasing and programming an organic memory device and methods of operating and fabricating

ABSTRACT

An organic memory cell made of two electrodes with a selectively conductive media between the two electrodes is disclosed. The selectively conductive media contains an organic layer and passive layer. The selectively conductive media is programmed by applying bias voltages that program a desired impedance state for a memory cell. The desired impedance state represents one or more bits of information and the memory cell does not require constant power or refresh cycles to maintain the desired impedance state. Furthermore, the selectively conductive media is read by applying a current and reading the impedance of the media in order to determine the impedance state of the memory cell. Methods of making the organic memory devices/cells, methods of using the organic memory devices/cells, and devices such as computers containing the organic memory devices/cells are also disclosed.

FIELD OF INVENTION

[0001] The present invention relates generally to organic memory devicesand, in particular, to erasing, programming, fabricating and utilizingorganic memory devices containing an organic polymer.

BACKGROUND OF THE INVENTION

[0002] The volume, use and complexity of computers and electronicdevices are continually increasing. Computers consistently become morepowerful, new and improved electronic devices are continually developed(e.g., digital audio players, video players). Additionally, the growthand use of digital media (e.g., digital audio, video, images, and thelike) have further pushed development of these devices. Such growth anddevelopment has vastly increased the amount of informationdesired/required to be stored and maintained for computer and electronicdevices.

[0003] Generally, information is stored and maintained in one or more ofa number of types of storage devices. Storage devices include long termstorage mediums such as, for example, hard disk drives, compact diskdrives and corresponding media, digital video disk (DVD) drives, and thelike. The long term storage mediums typically store larger amounts ofinformation at a lower cost, but are slower than other types of storagedevices. Storage devices also include memory devices which are often,but not always, short term storage mediums. Memory devices tend to besubstantially faster than long term storage mediums. Such memory devicesinclude, for example, dynamic random access memory (DRAM), static randomaccess memory (SRAM), double data rate memory (DDR), flash memory, readonly memory (ROM), and the like. Memory devices are subdivided intovolatile and non-volatile types. Volatile memory devices generally losetheir information if they lose power and typically require periodicrefresh cycles to maintain their information. Volatile memory devicesinclude, for example, random access memory (RAM), DRAM, SRAM and thelike. Non-volatile memory devices maintain their information whether ornot power is maintained to the devices. Non-volatile memory devicesinclude, but are not limited to, ROM, programmable read only memory(PROM), erasable programmable read only memory (EPROM), flash memory andthe like. Volatile memory devices generally provide faster operation ata lower cost as compared to non-volatile memory devices.

[0004] Memory devices generally include arrays of memory cells. Eachmemory cell can be accessed or “read”, “written”, and “erased” withinformation. The memory cells maintain information in an “off” or an“on” state (e.g., are limited to 2 states), also referred to as “0” and“1”. Typically, a memory device is addressed to retrieve a specifiednumber of byte(s) (e.g., 8 memory cells per byte). For volatile memorydevices, the memory cells must be periodically “refreshed” in order tomaintain their state. Such memory devices are usually fabricated fromsemiconductor devices that perform these various functions and arecapable of switching and maintaining the two states. The devices areoften fabricated with inorganic solid state technology, such as,crystalline silicon devices. A common semiconductor device employed inmemory devices is the metal oxide semiconductor field effect transistor(MOSFET).

[0005] The use of portable computer and electronic devices has greatlyincreased demand for non-volatile memory devices. Digital cameras,digital audio players, personal digital assistants, and the likegenerally seek to employ large capacity non-volatile memory devices(e.g., flash memory, smart media, compact flash, . . . ).

[0006] Because of the increasing demand for information storage, memorydevice developers and manufacturers are constantly attempting toincrease storage capacity for memory devices (e.g., increase storage perdie or chip). A postage-stamp-sized piece of silicon may contain tens ofmillions of transistors, each transistor as small as a few hundrednanometers. However, silicon-based devices are approaching theirfundamental physical size limits. Inorganic solid state devices aregenerally encumbered with a complex architecture which leads to highcost and a loss of data storage density. The volatile semiconductormemories based on inorganic semiconductor material must constantly besupplied with electric current with a resulting heating and highelectric power consumption in order to maintain stored information.Non-volatile semiconductor devices have a reduced data rate andrelatively high power consumption and large degree of complexity.

[0007] Moreover, as the size of inorganic solid state devices decreasesand integration increases, sensitivity to alignment tolerances increasesmaking fabrication markedly more difficult. Formation of features atsmall minimum sizes does not imply that the minimum size can be used forfabrication of working circuits. It is necessary to have alignmenttolerances which are much smaller than the small minimum size, forexample, one quarter the minimum size.

[0008] Scaling inorganic solid state devices raises issues with dopantdiffusion lengths. As dimensions are reduced, the dopant diffusionlengths in silicon are posing difficulties in process design. In thisconnection, many accommodations are made to reduce dopant mobility andto reduce time at high temperatures. However, it is not clear that suchaccommodations can be continued indefinitely.

[0009] Applying a voltage across a semiconductor junction (in thereverse-bias direction) creates a depletion region around the junction.The width of the depletion region depends on the doping levels of thesemiconductor. If the depletion region spreads to contact anotherdepletion region, punch-through or uncontrolled current flow, may occur.

[0010] Higher doping levels tend to minimize the separations required toprevent punch-through. However, if the voltage change per unit distanceis large, further difficulties are created in that a large voltagechange per unit distance implies that the magnitude of the electricfield is large. An electron traversing such a sharp gradient may beaccelerated to an energy level significantly higher than the minimumconduction band energy. Such an electron is known as a hot electron, andmay be sufficiently energetic to pass through an insulator, leading toirreversibly degradation of a semiconductor device.

[0011] Scaling and integration makes isolation in a monolithicsemiconductor substrate more challenging. In particular, lateralisolation of devices from each other is difficult in some situations.Another difficulty is leakage current scaling. Yet another difficulty ispresented by the diffusion of carriers within the substrate; that isfree carriers can diffuse over many tens of microns and neutralize astored charge. Thus, further device shrinking and density increasing maybe limited for inorganic memory devices. Furthermore, such deviceshrinkage for inorganic non-volatile memory devices while meetingincreased performance demands is particularly difficult, especiallywhile maintaining low costs.

SUMMARY OF THE INVENTION

[0012] The following is a summary of the invention in order to provide abasic understanding of some aspects of the invention. This summary isnot intended to identify key/critical elements of the invention or todelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

[0013] Systems and methods are provided for fabricating organic memorydevices and employing organic memory devices. The organic memory devicesutilize an organic conductor that facilitates migration of charge (e.g.,electrons, holes). The present invention provides organic memory devicesthat possess at least one or more of the following: small size comparedto inorganic memory devices, capability to store multiple bits ofinformation, short resistance/impedance switch time, low operatingvoltages, low cost, high reliability, long life (thousands/millions ofcycles), capable of three dimensional packing, associated lowtemperature processing, light weight, high density/integration, andextended memory retention.

[0014] An organic memory cell comprised of two electrodes with aselectively conductive media between the two electrodes is disclosed.The selectively conductive media contains an organic conductor layer andone or more passive layers. The selectively conductive media isprogrammed (e.g., written) by applying bias voltages that programs adesired impedance state into the memory cell. The desired impedancestate represents one or more bits of information and does not require aconstant power supply or refresh cycles to maintain the desiredimpedance state. The impedance state of the selectively conductive mediais read by applying a current and then reading the impedance of theselectively conductive media. As with the written impedance state, theread impedance state represents one or more bits of information.Additionally, methods of fabricating the organic memory devices/cells,methods of using the organic memory devices/cells, and devices such ascomputers containing the organic memory devices/cells are alsodisclosed.

[0015] To the accomplishment of the foregoing and related ends, theinvention comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative aspectsand implementations of the invention. These are indicative, however, ofbut a few of the various ways in which the principles of the inventionmay be employed. Other objects, advantages and novel features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a 3-D diagram of an organic memory device in accordancewith an aspect of the present invention.

[0017]FIG. 2 is a block diagram of a passive layer that can be employedin an organic memory device in accordance with an aspect of the presentinvention.

[0018]FIG. 3 is a block diagram illustrating an organic polymer layerformed by a CVD process in accordance with an aspect of the presentinvention.

[0019]FIG. 4 is a block diagram illustrating another organic polymerlayer formed by a CVD process in accordance with an aspect of thepresent invention.

[0020]FIG. 5 is a block diagram of yet another organic polymer layerformed by a CVD process in accordance with an aspect of the presentinvention.

[0021]FIG. 6 is a graph depicting the effect of an intrinsic electricfield on an interface between a passive layer and an organic polymerlayer in accordance with an aspect of the present invention.

[0022]FIG. 7 is a graph illustrating charge carrier distribution of anexemplary memory cell in accordance with an aspect of the presentinvention.

[0023]FIG. 8 is a graph illustrating charge carrier distribution of anexemplary memory cell in accordance with an aspect of the presentinvention.

[0024]FIG. 9 is a graph illustrating charge carrier distribution of anexemplary memory cell in accordance with an aspect of the presentinvention.

[0025]FIG. 10 is a graph illustrating charge carrier distribution of anexemplary memory cell in accordance with an aspect of the presentinvention.

[0026]FIG. 11 is a graph illustrating charge carrier concentration atthe interface of an exemplary memory cell in accordance with an aspectof the present invention.

[0027]FIG. 12 is a graph illustrating charge carrier concentration atthe interface of an exemplary memory cell in accordance with an aspectof the present invention.

[0028]FIG. 13 is a block diagram depicting an organic memory device invarious states in accordance with an aspect of the present invention.

[0029]FIG. 14 is a graph illustrating I-V characteristics for an organicmemory device in accordance with an aspect of the present invention.

[0030]FIG. 15 is a three dimensional view of an organic memory device inaccordance with an aspect of the present invention.

[0031]FIG. 16 is a flow diagram illustrating a method of fabricating anorganic memory device in accordance with an aspect of the presentinvention.

[0032]FIG. 17 is a flow diagram depicting a method of operating anorganic memory device in accordance with an aspect of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] The following is a detailed description of the present inventionmade in conjunction with the attached figures, wherein like referencenumerals will refer to like elements throughout.

[0034] The present invention provides an organic memory device that canoperate as a non-volatile memory device. The cells of the organic memorydevice are operative to be of two or more states corresponding tovarious levels of impedance. These states are set by applying a biasvoltage and then the cells remain in their respective states untilanother voltage, in reverse bias, is applied. The cells maintain theirstates with or without power (e.g., non-volatile) and can be read eitherelectrically or optically by measuring injection current or lightemission. The organic memory device of the present invention facilitatesincreases in device density whilst also increasing device performancerelative to conventional inorganic memory device.

[0035] Additionally, the organic memory device of the present inventionemploys electronic stimulation (e.g., flow of electrons and holes)instead of ions and/or electric fields. Thus, the organic memory devicecan have better performance and/or a quicker response to changes instimuli as compared to other types of memory devices.

[0036] Referring to FIG. 1, a 3-D diagram of an organic memory device inaccordance with an aspect of the present invention is depicted. Thememory device includes a first electrode 104, a passive layer 106, anorganic polymer layer 108, and a second electrode 110. The diagram alsoillustrates a voltage source 102 connected to the first electrode 104and the second electrode 110 that applies a voltage on the firstelectrode 104 and the second electrode 110.

[0037] The first electrode 104 and the second electrode 110 arecomprised of a conductive material such as, aluminum, chromium, copper,germanium, gold, magnesium, manganese, indium, iron, nickel, palladium,platinum, silver, titanium, zinc, alloys thereof, indium-tin oxide,polysilicon, doped amorphous silicon, metal silicides, and the like.Exemplary alloys that can be utilized for the conductive materialinclude Hastelloy®, Kovar®, Invar, Monel®, Inconel®, brass, stainlesssteel, magnesium-silver alloy, and various other alloys.

[0038] The thickness of the first electrode 104 and the second electrode110 can vary depending on the implementation and the memory device beingconstructed. However, some exemplary thickness ranges include about 0.01μm or more and about 10 μm or less, about 0.05 μm or more and about 5 μmor less, and/or about 0.1 μm or more and about 1 μm or less.

[0039] The organic layer 108 and the passive layer 106 are collectivelyreferred to as a selectively conductive media or selectively conductivelayer. The conductive properties of this media (e.g., conductive,non-conductive, semi-conductive) can be modified in a controlled mannerby applying various voltages across the media via the electrodes 104 and110.

[0040] The organic layer 108 is comprised of a conjugated organicmaterial, such as a small organic molecule and a conjugated polymer. Ifthe organic layer is polymer, a polymer backbone of the conjugatedorganic polymer may extend lengthwise between the electrodes 104 and 110(e.g., generally substantially perpendicular to the inner, facingsurfaces of the electrodes 104 and 110). The conjugated organic moleculecan be linear or branched such that the backbone retains its conjugatednature. Such conjugated molecules are characterized in that they haveoverlapping π orbitals and that they can assume two or more resonantstructures. The conjugated nature of the conjugated organic materialscontributes to the controllably conductive properties of the selectivelyconductive media.

[0041] In this connection, the conjugated organic material has theability to donate and accept charges (holes and/or electrons).Generally, the conjugated organic molecule has at least two relativelystable oxidation-reduction states. The two relatively stable statespermit the conjugated organic polymer to donate and accept charges andelectrically interact with the conductivity facilitating compound.

[0042] The organic material may be cyclic or acyclic. For some cases,such as organic polymers, the organic material self assembles betweenthe electrodes during formation or deposition. Examples of conjugatedorganic polymers include one or more of polyacetylene (cis or trans);polyphenylacetylene (cis or trans); polydiphenylacetylene; polyaniline;poly(p-phenylene vinylene); polythiophene; polyporphyrins; porphyrinicmacrocycles, thiol derivatized polyporphyrins; polymetallocenes such aspolyferrocenes, polyphthalocyanines; polyvinylenes; polystiroles; andthe like. Additionally, the properties of the organic material can bemodified by doping with a suitable dopant (e.g., salt). A more detaileddiscussion of the composition of the organic layer 108 is describedinfra.

[0043] The organic layer 108 has a suitable thickness that depends uponthe chosen implementations and/or the memory device being fabricated.Some suitable exemplary ranges of thickness for the organic polymerlayer 108 are about 0.001 μm or more and about 5 μm or less, about 0.01μm or more and about 2.5 μm or less, and about a thickness of about 0.05μm or more and about 1 μm or less.

[0044] The organic layer 108 can be formed via a number of suitabletechniques. One suitable technique that can be utilized is a spin-ontechnique which involves depositing a mixture of the material and asolvent, and then removing the solvent from the substrate/electrode.Another suitable technique is chemical vapor deposition (CVD). CVDincludes low pressure chemical vapor deposition (LPCVD), plasma enhancedchemical vapor deposition (PECVD), and high density chemical vapordeposition (HDCVD). It is not typically necessary to functionalize oneor more ends of the organic molecule in order to attach it to anelectrode/passive layer. Sometime it may have a chemical bond formedbetween the conjugated organic polymer and the passive layer 106.

[0045] The passive layer 106 contains at least one conductivityfacilitating compound that contributes to the controllably conductiveproperties of the selectively conductive media. The conductivityfacilitating compound has the ability to donate and accept charges(holes and/or electrons). Generally, the conductivity facilitatingcompound has at least two relatively stable oxidation-reduction states.The two relatively stable states permit the conductivity facilitatingcompound to donate and accept charges and electrically interact with theorganic layer 108. The particular conductivity facilitating compoundemployed is selected so that the two relatively stable states match withthe two relatively stable states of the conjugated organic molecule ofthe layer 108.

[0046] The passive layer 106 is operative to transport charge from thefirst electrode 104 to the interface between the organic layer 108 andthe passive layer 106. Additionally, the passive layer 106 facilitatescharge carrier (e.g., electrons or holes) injection into the organiclayer 108 and increases the concentration of the charge carrier in theorganic layer resulting in a modification of the conductivity of theorganic layer 108. Furthermore, the passive layer 106 can also storeopposite charges in the passive layer 106 in order to balance the totalcharge of the device 100.

[0047] The passive layer 106 can in some instances act as a catalystwhen forming the organic layer 108. In this connection, the backbone ofthe conjugated organic molecule may initially form adjacent the passivelayer 106, and grow or assemble away and substantially perpendicular tothe passive layer surface. As a result, the backbones of the conjugatedorganic molecule may be self aligned in a direction that traverses thetwo electrodes.

[0048] Examples of conductivity facilitating compounds that may make upthe passive layer 106 include one or more of copper sulfide (CU₂S, CuS),copper oxide (CuO, Cu₂O), manganese oxide (MnO₂), titanium dioxide(TiO₂), indium oxide (I₃O₄), silver sulfide (Ag₂S, AgS), , iron oxide(Fe₃O₄), and the like. The passive layer 106 may be grown usingoxidation techniques, formed via gas phase reactions, or depositedbetween the electrodes.

[0049] The passive layer 106 has a suitable thickness that can varybased on the implementation and/or memory device being fabricated. Someexamples of suitable thicknesses for the passive layer 106 are asfollows: a thickness of about 2 Å or more and about 0.1 μm or less, athickness of about 10 Å or more and about 0.01 μm or less, and athickness of about 50 Å or more and about 0.005 μm or less.

[0050] In order to facilitate operation of the organic memory device,the organic layer 108 is generally thicker than the passive layer 106.In one aspect, the thickness of the organic layer is from about 0.1 toabout 500 times greater than the thickness of the passive layer. It isappreciated that other suitable ratios can be employed in accordancewith the present invention.

[0051] The organic memory device, like conventional memory devices, canhave two states, a conductive (low impedance or “on”) state ornon-conductive (high impedance or “off”) state. However, unlikeconventional memory devices, the organic memory device is able tohave/maintain a plurality of states, in contrast to a conventionalmemory device that is limited to two states (e.g., off or on). Theorganic memory device can employ varying degrees of conductivity toidentify additional states. For example, the organic memory device canhave a low impedance state, such as a very highly conductive state (verylow impedance state), a highly conductive state (low impedance state), aconductive state (medium level impedance state), and a non-conductivestate (high impedance state) thereby enabling the storage of multiplebits of information in a single organic memory cell, such as 2 or morebits of information or 4 or more bits of information (e.g., 4 statesproviding 2 bits of information, 8 states providing 3 bits ofinformation . . . ).

[0052] During typical device operation, electrons flow from the secondelectrode 110 through the selectively conductive media to the firstelectrode 104 based on a voltage applied to the electrodes by thevoltage source 102 if the organic layer is n-type conductor.Alternately, holes flow from the first electrode 104 to second electrode110 if the organic layer 108 is p-type conductor, or both electrons andholes flow in the organic layer if it can be both n and p type withproper energy band match with 106 and 110. As such, current flows fromthe first electrode 104 to the second electrode 110 via the selectivelyconductive media.

[0053] Switching the organic memory device to a particular state isreferred to as programming or writing. Programming is accomplished byapplying a particular voltage (e.g., 9 volts, 2 volts, 1 volts, . . . )across the selectively conductive media via the electrodes 104 and 110.The particular voltage, also referred to as a threshold voltage, variesaccording to a respective desired state and is generally substantiallygreater than voltages employed during normal operation. Thus, there istypically a separate threshold voltage that corresponds to respectivedesired states (e.g., “off”, “on”. . . ). The threshold value variesdepending upon a number of factors including the identity of thematerials that constitute the organic memory device, the thickness ofthe various layers, and the like. The voltage supply 102 is controllablyemployed to apply the threshold voltage in this aspect of the invention.However, other aspects of the invention can utilize other means to applythreshold voltages.

[0054] Generally speaking, the presence of an external stimuli such asan applied electric field that exceeds a threshold value (“on” state)permits an applied voltage to write, read, or erase informationinto/from the organic memory cell; whereas the absence of the externalstimuli that exceeds a threshold value (“off” state) prevents an appliedvoltage to write or erase information into/from the organic memory cell.

[0055] To read information from the organic memory device, a voltage orelectric field (e.g., 2 volts, 1 volts, 0.5 volts) is applied via thevoltage source 102. Then, an impedance measurement is performed which,therein determines which operating state the memory device is in (e.g.,high impedance, very low impedance, low impedance, medium impedance, andthe like). As stated supra, the impedance relates to, for example, “on”(e.g., 1) or “off” (e.g., 0) for a dual state device or to “00”, “01”,“10”, or “11” for a quad state device. It is appreciated that othernumbers of states can provide other binary interpretations. To eraseinformation written into the organic memory device, a negative voltageor a polarity opposite the polarity of the writing signal that exceeds athreshold value is applied.

[0056]FIG. 2 is a block diagram that depicts fabrication of a passivelayer 200 in accordance with an aspect of the present invention. ACu_(y)S layer is formed by a gas phase reaction operation. A first layer206 is formed that comprises Cu. A second layer 204 is formed on thefirst layer. The second layer comprises Cu_(y)S (e.g., Cu₂S, CuS ormixture thereof) and has a thickness of about 20 Å or more. A thirdlayer 202 is formed on the second layer 204. The third layer 202contains Cu₂O, and/or CuO and generally has a thickness of about 10 Å orless. It is appreciated that alternate aspects of the invention canemploy suitable variations in composition and thickness and still be inaccordance with the present invention.

[0057]FIG. 3 is a block diagram illustrating an organic layer 300 formedby a chemical vapor deposition (CVD) process in accordance with anaspect of the present invention. The organic layer 300 is formed via agas phase reaction process. Typically, the organic layer 300 is formedin contact with a passive layer and an electrode. The organic layer 300is comprised of polymer polydiphenylacetylene (DPA). This polymer layer,as shown in FIG. 3, is fabricated to be about 65 Å thick.

[0058] Turning now to FIG. 4, a block diagram depicting another organiclayer 400 formed from a CVD process in accordance with an aspect of thepresent invention is illustrated. Once again, the organic layer 402 isformed via a gas phase reaction process. The organic layer 402 is formedin contact with a passive layer and an electrode. The organic polymerlayer 402 is comprised of polymer polyphenylacetylene (PPA). Referringto FIG. 5, a block diagram of another organic layer 500 formed by spincoating in accordance with an aspect of the present invention isillustrated. The organic layer 500 is formed via a spin coating process,instead of a gas phase reaction process. The organic layer 500 is formedin contact with a passive layer and an electrode. The organic layer 500is comprised substantially of PPA and has a thickness of about 1000 Å.

[0059] Experimental results tend to show that organic layers formed viaspin coating yield a more reliable polymer layer than polymer layersformed via CVD. This may be due to the presence of oxygen and lack ofcontrol of heat generated by polymerization under CVD. It is appreciatedthat controlling heat and oxygen during polymerization for CVD processescan improve the resulting polymer layer. Additionally, organic layerscreated via CVD are generally thinner than those created with othermethods.

[0060] It is appreciated that various alternatives to and variations ofthe layers described in FIG. 2-5 can be employed in accordance with thepresent invention.

[0061] The passive layer (e.g., CuS) employed in organic memory devicesplay an important role. Its presence significantly improves theconductivity of the organic layer. This characteristic is at leastpartially a function of the following: charge carrier generated by CuS,build up of a charge depletion layer, charge carrier distribution inorganic material, and memory loss due to charge carrier redistributionafter reversing electric field. The discussion infra describes andillustrates charge carrier concentration and models behavior of organicmemory devices.

[0062] In the following example, conductive polymer is used as organicmaterial, and CuS is used as passive layer material. With respect tocharge carrier generation, the copper in CuS is at its highest oxidationstate Cu(II). It has relatively strong capability to gain electrons froma contacting polymer and yields the following equation:

Cu(II)S+Polymer→Cu(I)S⁻+Polymer⁺  (1)

[0063] The consequence is that an intrinsic field is produced due to thecharges accumulated on the interface between CuS and polymer. This isshown in FIG. 6, which is a graph depicting the effect of an intrinsicelectric field on an interface between Cu(y)S and a polymer is provided.The oxidized polymer (Polymer⁺) is the charge carrier when externalfield is applied. The conductivity of polymer is determined by itsconcentration and its mobility.

σ=qp μ  (2)

[0064] Where q is the charge of the carrier, p is carrier concentrationand p is the mobility.

[0065] Referring now to the charge depletion layer, employing a similarconcept as applied with respect to semiconductors, a potential functioncan be expressed as:

V(x)=qN _(p)(d _(p) x−x ²/2)/ε  (3)

[0066] where N_(p) is the average concentration of charge carrier, ε isthe dielectric constant of the polymer, and d_(p) is the width of thecharge depletion. N_(p) can be obtained by employing the followingequation: $\begin{matrix}{d_{p} = \lbrack \frac{2\quad {ɛ( {V_{b} \pm V} )}}{q\quad N_{p}} \rbrack^{1/2}} & (4)\end{matrix}$

[0067] where V is the external field voltage applied. For forwardvoltage, it is “−” sign. For the reverse voltage, it is “+” sign. Thevoltage function of Eq. (3) can be approximated to simplify thederivation.

[0068] With respect to charge carrier distribution, like p-doping of asemiconductor, two processes typically take place in the electric field.This flux can be expressed as: $\begin{matrix}{J = {{{- q}\quad D\frac{p}{x}} + {q\quad \mu \quad p\quad E}}} & (5)\end{matrix}$

[0069] where D is diffusion constant of the charge carrier, and E is theelectric field at x. If there is no current, the carrier distributionis:

p(x)=p(0)exp([(V(0)−V(x))/Vt])  (6)

[0070] where p(0) is the concentration, V(0) is voltage at the interfacerespectively, and V_(t)=kT/q.

[0071] When forward voltage is so large that the current flux J>0, theanalytical equation can be derived for steady state flow with someassumption for the voltage distribution in the cell. Overall, underforward voltage, the charge distribution p(x) is an increase function ofx. When reverse voltage is applied, V(x)>V₀, the charge concentration isa decrease function of x.

[0072] The final characteristic, retention time, refers to the fact thata forward voltage produces more charge carrier and the charge carrieraccumulates more on the other end of the passive (CuS) layer (away fromthe organic polymer). However, this charge carrier concentration will beset back once the voltage is removed, which includes two processes:charge carrier diffusion toward the CuS layer and charge carrierrecombination on the interface.

[0073] Fick's Law can describe the 1st process, charge carrier diffusiontoward the CuS layer.

[0074] The charge carrier recombination can be described as follows:

Cu(I)S⁻+Polymer⁺→Cu(II)S+Polymer  (7)

[0075] The retention time is the time required to redistribute thecharge carrier to the original state. It is likely that the reactionrate is relatively faster than diffusion rate. Therefore, the retentiontime can be substantially determined by the diffusion process only.

[0076] An exemplary memory cell is considered herein with respect to theequations 1-9 discussed supra and illustrated in FIG. 7-12. Theexemplary cell is considered with parameters intrinsic voltageV_(b)=0.02V, equilibrium constant K_(eq)=2.17×10⁻⁴, concentration of CuSand Polymer at interface [Polymer]₀=[CuS]₀=10²³/cm³, polymer thicknessd=5×10⁻⁵ cm (0.5 um), and CuS thickness d_(CuS)=5×10^(−7 cm ()0.005 um).Six typical cases are calculated to illustrate electrical operation ofan organic memory device in accordance with an aspect of the presentinvention.

[0077]FIG. 7 depicts a graph 700 of charge carrier distribution 701 ofthe exemplary memory cell as a function of distance from the CuS andorganic polymer interface in accordance with an aspect of the invention.The charge carrier concentration 701 is shown as being a decreasingfunction of distance (x) from the interface. This graph 700 assumes anexternal voltage V=0 and a current J=0. The charge carrier concentration701 is derived utilizing Eq. 6 with a constant field assumption.However, the points shown are independent of the constant fieldassumption.

[0078] Turning now to FIG. 8, another graph 800 illustrating chargecarrier distribution 801 for the exemplary organic memory cell isdepicted in accordance with an aspect of the present invention. For thisgraph 800, parameters are set as follows:

[0079] forward voltage=0.12V and current flux J=0. The CuS end has ahigher voltage than the other end (organic polymer). This drives thecharge carrier away from CuS layer and leads to charge carrierconcentration that has an increase function of x. Even at lowestconcentration p(0), it is not a small value for this case (e.g., itsvalue is 3.32×10¹⁹/cm³ for the case shown in FIG. 8). This explains whythe polymer is a good conductor when forward voltage is applied. Again,it is Eq. 6 with constant electric field model used for the plot. Thepoints demonstrated are independent of constant electric fieldassumption.

[0080]FIG. 9 depicts yet another graph 900 of charge carrierdistribution 901 of the exemplary memory cell as a function of distancefrom the CuS and organic polymer interface in accordance with an aspectof the invention. For this graph, the parameters are set such that thereverse voltage=0.28V and the current J=0. With reversed voltage, thecharge carrier is concentrated at the CuS polymer interface and dropsquickly to small concentration when it is away from the interface, whichdescribes why the organic memory cell becomes non-conductive when highreversed voltage applied. Again, Eq. 6 with constant electric fieldmodel is assumed for the plot. The points demonstrated are independentof this assumption.

[0081] Referring now to FIG. 10, another graph 1000 that depicts chargecarrier distribution 1001 of the exemplary memory cell as a function ofdistance in accordance with an aspect of the present invention isprovided. For this graph 1000, parameters are set as follows: forwardvoltage=0.52V and current flux J>0 (p_(J)=10¹⁸/cm³). When current fluxJ>0, the charge carrier is still an increase function of x because theforward voltage drives the charge carrier away from CuS interface. Oneimportant point is that the lowest concentration p(x) is at interface.

[0082]FIG. 11 depicts yet another graph 1100 of charge carrierconcentration at interface 1101 of the exemplary memory cell as functionof forward voltage V. For this graph, the parameters are set such thatJ>0 (p_(J)=10^(18/cm) ³) and assumes a constant electric field model.This model assumes the electric field in the cell is constant.Therefore, the voltage V(x) is described as a linear function. Thismodel is applicable when the diffusion constant of the polymer is smalland there is constant electric resistance. With this model, the chargecarrier concentration at interface is derived as function of voltage. Itis noted that p₀(V) tends to be constant after forward voltage is largeenough and the current is controlled by the charge carrier not chargeinjection at the interface. As such, p(0) can be rewritten as:$\begin{matrix}{{p(0)} = {\frac{1}{2}\{ {{- {K_{eq}\lbrack{Polymer}\rbrack}_{0}} + \sqrt{( {K_{eq}\lbrack{Polymer}\rbrack}_{0} )^{2} + \frac{4d_{CuS}{{K_{eq}\lbrack{Polymer}\rbrack}_{0}\lbrack{CuS}\rbrack}_{0}}{d}}} \}}} & (10)\end{matrix}$

[0083] This Eq. 10 shows that limiting p(0) is an increase function ofthickness ratio between CuS layer and polymer layer.

[0084]FIG. 12 illustrates another graph 1200 that depicts charge carrierconcentration at the interface 1201 of the exemplary memory cell asfunction of forward voltage Vin accordance with an aspect of the presentinvention is provided. For this graph 1200, p(0) is a function offorward voltage, current J, which may or may not be >0, and a steppotential function model. This model assumes the voltage V(x) functioncan be described by a step function. The model is applicable when thediffusion constant of the polymer is very large. Therefore, the electricresistance in the cell is trivial. With this model, the charge carrierconcentration at interface is derived as the function of voltage. It isnoted that in FIG. 12 that p₀(V) tends to be zero after forward voltageis large enough. When the charge carrier at the interface controls thecurrent flux, this value is a function of voltage. This zero limitbehavior is due to the interface boundary limit set by the reaction (1).Basically, the fast charge carrier transportation from the interface tothe other end reaches the supply limit. Thus, the limiting p(0) is alsorewritten as: $\begin{matrix}{{p(0)} = {\frac{1}{2}\{ {{- {K_{eq}\lbrack{Polymer}\rbrack}_{0}} + \sqrt{( {K_{eq}\lbrack{Polymer}\rbrack}_{0} )^{2} + \frac{4d_{CuS}{{K_{eq}\lbrack{Polymer}\rbrack}_{0}\lbrack{CuS}\rbrack}_{0}}{d\lbrack {{\exp \frac{{V(0)} - V}{V_{t}}} - \frac{{V(0)} - V}{V_{t}}} \rbrack}}} \}}} & (11)\end{matrix}$

[0085] Again p(0) is an increase function of thickness ratio between CuSlayer and polymer layer.

[0086] Regarding the above discussion, it is important to note that theflux measured is determined by charge carrier drift when limiting fluxis in the polymer. Under constant electric field assumption, thefunction to describe the charge carrier concentration is p(x)·p_(J)=p(0)is met when the polymer determines limiting flux since the lowestconcentration in the cell is at the interface. This condition results ina constant p(x). This means the diffusion contribution to the flux inEq. 5 is zero. Under step potential assumption, another function isemployed to describe the charge carrier concentration p(x). The initialcharge carrier concentration p(0) has a relatively substantially smallervalue than other regions. Therefore, J is still determined by p(0).Another point that is noted regards boundary conditions. Unlikesemiconductors, it is just applicable to the concentration at interface,not everywhere. This boundary condition limits the total amount of thecharge carrier produced in the cell.

[0087] The equations supra (E.q. 1-7) and the FIGS. 9-12 describe andmodel behavior of organic memory devices. This model can be employed toexplain measured data and can be for other passive layer materials asidefrom CuS. Additionally, the model can be used to think about how toimprove retention and response time and to design the other devices suchas transistor. Further, the model can be employed to develop variousthreshold voltages that set conductivity levels (e.g., set states), readconductivity levels and erase the conductivity levels thus performingmemory device operations of writing or programming, reading and erasing.

[0088]FIG. 13 is a block diagram that illustrates an organic memorydevice 1300 in various states in accordance with an aspect of thepresent invention. The device 1300 is depicted in a first “off” state1301, an “on” state 1302, and a second “off” state 1303. It isappreciated that memory devices formed in accordance with the presentinvention can have other states than those depicted in FIG. 13. Theorganic memory device 1300 comprises a top electrode 1304, a bottomelectrode 1306 and a selectively conductive layer 1308 comprising anorganic layer (e.g., PPA) and at least one passive layer (e.g., CuS).

[0089] In the first off state 1301, a positive charge 1310 collects inthe selectively conductive layer 1308 near the bottom electrode 1306. Inthe on state 1302, the positive charge 1310 is uniformly distributedthereby indicating an on state. In the second off state 1303, thepositive charge collects in the selectively conductive layer 1308 nearthe top electrode 1304.

[0090]FIG. 14 is a graph 1400 that illustrates I-V characteristics forthe memory device 1300 described with respect to FIG. 13. It can be seenthat from state 1, which indicates “off”, the device can be modified tobe in state 2, which indicates “on”, by applying a positive voltage of2V. Additionally, it can be seen that whilst in state 1, the organicmemory device has a high impedance and low conductance. Subsequently,the device 1300 can be modified to change from state 2 to state 1 byapplication of a negative voltage, therein causing a reverse currentuntil the state 1 is obtained.

[0091] Referring to FIG. 15, a three dimensional view of an organicmemory device 1500 containing a plurality of organic memory cells inaccordance with an aspect of the invention is shown. The organic memorydevice 1500 contains a plurality of first electrodes 1502, a pluralityof second electrodes 1504, and a plurality of memory cell layers 1506.Between the respective first and second electrodes are the controllablyconductive media (not shown). The plurality of first electrodes 1502 andthe plurality of second electrodes 1504 are shown in substantiallyperpendicular orientation, although other orientations are possible. Thethree dimensional microelectronic organic memory device is capable ofcontaining an extremely high number of memory cells thereby improvingdevice density. Peripheral circuitry and devices are not shown forbrevity.

[0092] The organic memory cells/devices are useful in any devicerequiring memory. For example, the organic memory devices are useful incomputers, appliances, industrial equipment, hand-held devices,telecommunications equipment, medical equipment, research anddevelopment equipment, transportation vehicles, radar/satellite devices,and the like. Hand-held devices, and particularly hand-held electronicdevices, achieve improvements in portability due to the small size andlight weight of the organic memory devices. Examples of hand-helddevices include cell phones and other two way communication devices,personal data assistants, palm pilots, pagers, notebook computers,remote controls, recorders (video and audio), radios, small televisionsand web viewers, cameras, and the like.

[0093] In view of the foregoing structural and functional featuresdescribed above, methodologies in accordance with various aspects of thepresent invention will be better appreciated with reference to FIGS.16-17. While, for purposes of simplicity of explanation, themethodologies of FIGS. 16-17 is depicted and described as executingserially, it is to be understood and appreciated that the presentinvention is not limited by the illustrated order, as some aspectscould, in accordance with the present invention, occur in differentorders and/or concurrently with other aspects from that depicted anddescribed herein. Moreover, not all illustrated features may be requiredto implement a methodology in accordance with an aspect the presentinvention.

[0094]FIG. 16 illustrates a flow diagram of a method 1600 of fabricatingan organic memory device in accordance with an aspect of the invention.

[0095] A first electrode is formed on a substrate at 1602. The firstelectrode is comprised of a conductive material such as, aluminum,chromium, copper, germanium, gold, magnesium, manganese, indium, iron,nickel, palladium, platinum, silver, titanium, zinc, alloys thereof,indium-tin oxide, polysilicon, doped amorphous silicon, metal silicides,and the like. Exemplary alloys that can be utilized for the conductivematerial include Hastelloy®, Kovarg®, Invar, Monel®, Inconel®, brass,stainless steel, magnesium-silver alloy, and various other alloys. Thethickness of the first electrode can vary depending on theimplementation and the memory device being constructed. However, someexemplary thickness ranges include about 0.01 μm or more and about 10 μmor less, about 0.05 μm or more and about 5 μm or less, and/or about 0.1μm or more and about 1 μm or less.

[0096] After forming the first electrode, a passive layer is formed onthe first electrode layer at 1604. The passive layer contains at leastone conductivity facilitating compound that contributes to thecontrollably conductive properties of the selectively conductive media.The conductivity facilitating compound has the ability to donate andaccept charges (holes and/or electrons). Generally, the conductivityfacilitating compound has at least two relatively stableoxidation-reduction states. The two relatively stable states permit theconductivity facilitating compound to donate and accept charges andelectrically interact with the organic layer. The particularconductivity facilitating compound employed is selected so that the tworelatively stable states match with the two relatively stable states ofthe conjugated organic polymer of the organic polymer layer.

[0097] The passive layer can, in some instances, act as a catalyst forforming the organic layer requiring the passive layer to be formedbefore the organic layer. In this connection, the backbone of theconjugated organic molecule may initially form adjacent the passivelayer, and grow or assemble away and substantially perpendicular to thepassive layer surface. As a result, the backbones of the conjugatedorganic molecules are self aligned in a direction that traverses the twoelectrodes.

[0098] Examples of conductivity facilitating compounds that may make upthe 25 passive layer include one or more of the following: coppersulfide (Cu₂S, CuS), copper oxide (CuO, Cu₂O), manganese oxide (MnO₂),titanium dioxide (TiO₂), indium oxide (I₃O₄), silver sulfide (Ag₂S,AgS), gold sulfide (Au₂S, AuS), , iron oxide (Fe₃O₄), and the like. Thepassive layer 106 is typically grown using oxidation techniques, formedvia gas phase reactions, or deposited between the electrodes.

[0099] The passive layer has a suitable thickness that can varyaccording to the implementation and/or memory device being fabricated.Some examples of suitable thicknesses for the passive layer are asfollows: a thickness of about 2 Å or more and about 0.1 μm or less, athickness of about 10 Å or more and about 0.01 μm or less, and athickness of about 50 Å or more and about 0.005 μm or less.

[0100] Next, an organic layer is formed on the passive layer at 1606.The organic layer comprises a conjugated molecule(s). The organicpolymer layer is comprised of a conjugated organic polymer. Suchconjugated molecules are characterized in that they have overlapping πorbitals and that they can assume two or more resonant structures. Theorganic molecules may be cyclic or acyclic. During formation ordeposition, the organic molecule may self assemble between theelectrodes. Examples of conjugated organic materials include one or moreof polyacetylene (cis or trans); polyphenylacetylene (cis or trans);polydiphenylacetylene; polyaniline; poly(p-phenylene vinylene);polythiophene; polyporphyrins; porphyrinic macrocycles, thiolderivatized polyporphyrins; polymetallocenes such as polyferrocenes,polyphthalocyanines; polyvinylenes; polystiroles; and the like.Additionally, the properties of the polymer can be modified by dopingwith a suitable dopant (e.g., salt).

[0101] The organic layer is formed with a suitable thickness thatdepends upon the chosen implementations and/or the memory device beingfabricated. Some suitable exemplary ranges of thickness for the organiclayer are about 0.001 μm or more and about 5 μm or less, about 0.01 μmor more and about 2.5 μm or less, and about a thickness of about 0.05 μmor more and about 1 μm or less.

[0102] The organic layer can be formed via a number of suitabletechniques, some of which are described supra. One suitable techniquethat can be utilized is a spin-on technique which involves depositing amixture of the polymer/polymer precursor and a solvent, and thenremoving the solvent from the substrate/electrode. Another technique ischemical vapor deposition (CVD) optionally including a gas reaction, gasphase deposition, and the like. CVD includes low pressure chemical vapordeposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD),and high density chemical vapor deposition (HDCVD). It is not typicallynecessary to functionalize one or more ends of the organic molecule inorder to attach it to an electrode/passive layer.

[0103] In order to facilitate operation of the organic memory device,the organic layer is generally, but not always, substantially thickerthan the passive layer. As one example, the thickness of the organiclayer is from about 0.1 to about 500 times greater than the thickness ofthe passive layer. As another example, the thickness of the organiclayer is from about 25 to about 250 times greater than the thickness ofthe passive layer. It is appreciated that other suitable ratios can beemployed in accordance with the present invention.

[0104] The organic layer and the passive layer are collectively referredto as a selectively conductive media or selectively conductive layer.The conductive properties of this media (e.g., conductive,non-conductive, semi-conductive) are modified, in a controlled manner,by applying various voltages (e.g., bias voltages) across the media.

[0105] Finally, a second electrode is formed over the organic layer at1608. The second electrode is formed of a conductive material in amanner similar to that of the first electrode. The second electrode can,but is not required to, be formed of the same conductive material as thefirst electrode.

[0106] Turning now to FIG. 17, a flow diagram of a method 1700 ofoperating an organic memory device in accordance with the presentinvention is depicted. The method 1700 can be employed to operate anorganic memory device, such as that described with respect to FIG. 16.The operation of the device includes reading and writing information toand from the organic memory device. It is appreciated that the method1700 can operated on memory cells and arrays of memory cells within theorganic memory device.

[0107] The method 1700 begins at 1702 where a determination is made asto whether a read or a write operation is to be performed. For adetermination of a write operation at 1702, a desired impedance state isdetermined at 1704. The impedance state corresponds to a desired statevalue and/or desired information content for respective cell(s) (e.g.,0, 1, 11, 10, and the like). The desired state is one of a plurality ofavailable reference states or impedance levels for the device, where theavailable reference states indicate different information content. Abias voltage is then applied to the cell at 1706 in order to write thedesired state. The method 1700 then returns to 1702 where adetermination is made for a subsequent operation.

[0108] The desired information content is typically stored in theorganic memory device at addressable memory locations (e.g., cells) in aspecified number of bytes. However, unlike conventional memory devices,more than one bit of information can be written to a single memory cell,thus a byte of information can be stored into less than 8 memory cells.

[0109] On the determination of a read operation at 1702, the method 1700continues at 1708 where an injection current or voltage is applied inorder to measure impedance of the memory cell(s). It is appreciated thatalternate aspects of the invention can read the memory cell(s) by lightemission. The measured impedance is then compared with the availablereference impedance states in order to determine the impedance state at1710 in order to obtain the information content. As described supra, theimpedance state corresponds to a desired state value of a particularcell (e.g., 0, 1, 11, 10, and the like) and can thus provide one or morebits of information. The impedance state is one of the availablereference states for the memory cell(s), described supra.

[0110] As discussed supra, the desired information content is typicallystored in the organic memory device at addressable memory locations(e.g., cells) in a specified number of bytes. However, unlikeconventional memory devices, more than one bit of information can beread from a single memory cell.

[0111] It is appreciated that the above description of the method 1700has been somewhat simplified in order to facilitate understanding of thepresent invention. For example, cells of the memory device are accessedvia an addressing scheme in order to read and write memory locations.Additionally, the memory cell(s) can be erased (e.g., set to a defaultstate) by applying an appropriate voltage across the selectivelyconductive media of the memory device. The default state is generally avery highly conductive or very low conductive impedance state.

[0112] What have been described above are one or more aspects of thepresent invention. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the present invention, but one of ordinary skill in the artwill recognize that many further combinations and permutations of thepresent invention are possible. Accordingly, the present invention isintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the term“includes” is used in either the detailed description and the claims,such term is intended to be inclusive in a manner similar to the term“comprising.”

What is claimed is:
 1. An organic memory device comprising: a firstelectrode; a selectively conductive media formed on the first electrode,the selectively conductive media facilitating migration of charge, theselectively conductive media comprising an organic material, and thecharge comprising at least one of electrons and holes; and a secondelectrode, wherein a selected voltage is applied to the first electrodeand the second electrode in order to set an impedance state of theselectively conductive media.
 2. The device of claim 1, the selectedvoltage being one of a number of voltages that corresponds to theresulting impedance state.
 3. The device of claim 1, the impedance statebeing one of a number of available impedance states.
 4. The device ofclaim 3, the number of available impedance states corresponding torespective information content.
 5. The device of claim 1, the impedancestate representing more than one bit of information.
 6. The device ofclaim 1, wherein an injection current is applied to the first electrodeand the second electrode to read a current state of the organic memorydevice.
 7. The device of claim 6, the current state being one of anumber of available states.
 8. The device of claim 7, the availablestates representing one or more bits of information.
 9. The device ofclaim 1, the selectively conductive media comprising a passive layerformed on the first electrode and an organic polymer layer formed on thepassive layer.
 10. The device of claim 9, the passive layer comprising aplurality of individual passive layers.
 11. The device of claim 9, thepassive layer contains Cu_(y)S.
 12. The device of claim 9, the organiclayer being a conjugated organic material.
 13. The device of claim 9,the organic layer being selected from the group comprising:polyacetylene, polyphenylacetylene, polydiphenylacetylene, polyaniline,poly(p-phenylene vinylene), polythiophene, polyporphyrins, porphyrinicmacrocycles, thiol derivatized polyporphyrins, polymetallocenes,polyferrocenes, polyphthalocyanines, polyvinylenes, and polystiroles.14. The device of claim 1, the first electrode comprising a materialbeing selected from the group comprising aluminum, chromium, copper,germanium, gold, magnesium, manganese, indium, iron, nickel, palladium,platinum, silver, titanium, zinc, alloys thereof, indium-tin oxide,polysilicon, doped amorphous silicon, and metal silicides.
 15. Thedevice of claim 1, thicknesses of the first electrode and the secondelectrode being about 0.01 μm or more and about 10 μm or less.
 16. Thedevice of claim 9, the organic layer having a thickness of about 0.001μm or more and about 5 μm or less.
 17. The device of claim 9, athickness of the organic layer is about 1 to 500 times greater than athickness of the passive layer.
 18. A method of fabricating an organicmemory device comprising: forming a first electrode on a substrate;forming a passive layer on the first electrode; forming an organic layeron the passive layer, the organic layer being substantially thicker thanthe passive layer; and forming a second electrode on the organic layer.19. The method of claim 18, the organic layer formed via a chemicalvapor deposition process.
 20. The method of claim 18, the organic layerformed via a gas phase reaction process.
 21. The method of claim 18, theorganic layer formed via a spin coating process.
 22. The method of claim18, further comprising applying a first voltage to the first electrodeand the second electrode to set an impedance state of the memory device,the impedance state representing information content.
 23. The method ofclaim 18, further comprising applying a second voltage to the firstelectrode and the second electrode to determine an impedance state ofthe memory device, the impedance state representing information content.24. A system for operating an organic memory device comprising: meansfor determining impedance state(s) of one or more memory cells; andmeans for determining information content from the impedance state(s).25. The system of claim 24, further comprising: means for determiningdesired impedance state(s) for information content to be stored; andmeans for writing information content to one or more memory cells byprogramming the desired impedance state(s) into the one or more memorycells.
 26. An organic memory device comprising: a first electrode; asecond electrode; an organic media residing within an area between thefirst and second electrodes; and a passivation layer interposing thefirst electrode and the organic media, the passivation layerfacilitating migration of electrons and/or holes across the organicmedia.